1. Field of Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention is directed to the use of carbon monofluoride (CFx) in high pulse power cells containing a transition metal oxide such as silver vanadium oxide (SVO).
It has been discovered that when CFx material is prepared from highly structured carbon precursors, cell swelling during high current pulse discharge conditions is markedly reduced, and in some cases eliminated. Still more particularly, the present invention describes a lithium electrochemical cell designed for high rate discharge applications in which the cathode electrode preferably has a sandwich design of the configuration: SVO/current collector/CFx/current collector/SVO. Cells with this cathode electrode design are particularly applicable for powering implantable medical devices, such as cardiac defibrillators, requiring a relatively low electrical current for device monitoring functions interrupted from time to time by a high current pulse discharge for device activation.
2. Prior Art
U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present invention and incorporated herein by reference, describes a sandwiched cathode design for use in a high rate electrochemical cell. The sandwich cathode is composed of a first cathode active material of a relatively high energy density but of a relatively low rate capability, such as CFx, Ag2O2, and even SVO, sandwiched between two layers of current collector. This assembly is, in turn, sandwiched between two layers of a second cathode active material of a relatively high rate capability but of a relatively low energy density, such as SVO, copper silver vanadium oxide (CSVO) and MnO2. Significantly higher capacities are obtained from lithium cells having sandwich cathode designs of SVO/CFx/SVO relative to those of lithium cells using only SVO active material in a conventional cathode design. A conventional cathode design has the SVO active material contacted to both sides of an intermediate cathode current collection. In addition, the higher capacity of the present invention cell is achieved without sacrificing the cell""s power capability. Therefore, lithium cells constructed with a sandwich cathode electrode design are very good candidates as power sources for cardiac defibrillators and other implantable medical devices requiring a high power cell.
Other than cell capacity, an important consideration for an implantable medical device application is cell swelling during discharge. In order to prevent damage to device circuitry, enough void space must be left inside the powered device to accommodate this volume change. The more cell swelling, the more void space that must be reserved. Cell swelling, therefore, impacts the device total volume. In the field of implantable biomedical devices, a smaller total device volume is desired. Thus, in order to provide a more compact device design, it is desirable to minimize or eliminate cell swelling. Excessive cell swelling is also detrimental to the proper functioning of the implantable medical device and, consequently, to its safe use.
Carbon monofluoride, CFx is a cathode active material that has found wide spread use for low-weight lithium cells. In fact, Li/CFx cells are particularly useful for discharge applications requiring relatively low currents of about 1 microamperes to about 100 milliamperes. At these discharge rates, cell swelling is generally not observed. However, when Li/CFx cells having the cathode active material synthesized from petroleum coke are discharged under relatively high current applications, i.e., from about 15.0 mA/cm2 to about 35.0 mA/cm2, significant cell swelling is observed.
Cells powering implantable cardiac defibrillators are periodically pulse discharged under very high current densities of 15.0 mA/cm2 and higher. Therefore, when CFx is included in a sandwich cathode design as part of an electrode assembly powering an implantable medical device, such as a cardiac defibrillator, and the active material is synthesized from certain carbonaceous precursors such as petroleum coke, a potential cell swelling problem exists.
Accordingly, what is needed is a fluorinated carbon active material which is capable of being subjected to relatively high rate discharge conditions without appreciably swelling. Such a material would be very desirable for inclusion into a high rate cell powering an implantable medical device.
According to the present invention, the swelling of cells containing sandwich cathodes, such as of a SVO/CFx/SVO configuration, is significantly minimized by using a CFx material synthesized from carbon fibers, and mesophase carbon microbeads (MCMB). These carbonaceous materials are identified as those which result in the least amount of swelling in cells containing CFx as part of the cathode material relative to those synthesized from graphite or petroleum coke. This is especially important when the cell is being pulse discharged.
These and other aspects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and the appended drawing.